The present invention relates to an improved construction of an adapter for mounting an instant picture cassette to a camera, the back wall of which is removable, comprising an adapter plate which is fixedly mounted to the instant picture cassette and is carrying a back member corresponding to said removable back wall, the adapter plate and the back member carrying a fiber optic face plate arranged in an opening therein for transferring an image formed at an image plane of the camera to the plane of the film in the instant picture cassette.